Not sure there's exactly a "limit"... but having a really long username with no spacing (like, for instance "ThatOneDenverMooseGuy") causes posts to look kinda annoying... like the page is divided almost in half, with half of the page width taken up by the long username above the avy and the text of the post crowded into the other half (you can check one of his posts to see what I'm talking about ).